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You need a pal to deal with PAL When the PAL colour system was first introduced we all burnt a lot of midnight oil trying to bridge the gap from NTSC theory to PAL theory. While most of us felt we had succeeded, time and day-today practical work have taken their toll. Now, when this theory might help, our memory fails us. Nothing very exciting happened on my own bench this month, so the following story comes from my colleague J. L. in Tasmania. It tells how he encountered the need for some PAL theory. Here's J. L.'s story: There are some television sets that I don't like servicing. One such set is the Sharp model C1831X (and I can hear a lot of my colleagues saying "Hear, Hear!"). It's true that many of us have had trouble with this chassis yet others have had no trouble at all. It's almost as if the set has a mind of its own and doesn't like anyone wearing glasses, or a bald head, or false teeth. On the other hand, chaps with beards seem to have no trouble at all with this set, so maybe we should all grow long hair! There are a number of common problems in the vertical section of the C1831X which are fairly easy to cure, so long as you are careful. Unfortunately, as you slide the chassis out of the cabinet, it is all too easy to allow the contacts on the underside of the vertical oscillator chip to touch the metal rails on which the chassis moves. It doesn't seem to matter whether the power is on or off; you don't get a second chance and the chips are quite expensive. Another problem is with the wire-wrap interboard connections. You can only flex a single core wire so many times and if you are chas- ing a difficult fault it is easy to bend the wires once too often. They tend to break inside the insulation and you don't know until you switch on and some part of the set no longer works. I heard of one case where a vital earth link broke and somehow allowed the 200V rail to go to chassis through the 12V rail. It did dreadful things to every IC and transistor in the set! So with this sort of background to the C1831X, and the fact that my glasses steam up every time I see one, I am quite happy to concentrate on other models and leave this particular Sharp model to those who enjoy head-butting brick walls. Oh boy, a C1831X But sometimes you just can't refuse to do a job and the story that follows tells about one such exer- ,_.T\·HS 1\MG: SH~ C.A MC:., NOi" -ro C.OME. & FI~ i\-\~E-r, SU\ i'O ,A~E n- AWA'-{ FOR '""C:R ..... 52 SILICON CHIP .d ~ ~ ~- iHE. 'PlCtURe:.. WPS V ~ DARK & SH-€ C.OUL..'D ONL.'1 WlrrCH rr ITT" \\l\GH-r: ... cise. When a customer gives you an old set for junk there's a strong temptation to get it going again, if only to see if it can be done. And if it's a C1831X, then you're stuck with it! This particular C1831X belonged to a well-liked, elderly customer and I had attended to it a couple of times in the past. The faults were never very serious but I was always apprehensive whenever I heard her on the phone. This time she called me, not to come and fix the set but to take it away for her. It seems that a small insurance policy had matured and rather than bank the money and complicate her pension status, she bought a new set to replace the aging Sharp. It had been playing up a bit before it was retired. The picture was very dark and the owner could only watch it at night. She guessed that the picture tube had gone but that I might get some useful parts out of the rest of it. Hence the call. I wanted a Sharp C1831X like the legendary "hole in the head" but out of respect for a valued customer, and at the first convenient opportunity, I called to collect it. The last time I had seen the Sharp was some six months earlier and it had then displayed quite a good picture. I wondered if it really had dropped its bundle so suddenly or was there a more logical reason for the dark picture. Back in the workshop I fired it up and saw what I had suspected all along. The picture was truly low in brightness but the chroma was more or less normal. It had to be an open circuit bypass capacitor or feed resistor in the supply rail to the video output transistors. This kind of fault is common in circuits where the video output supply rail is bypassed with an electrolytic capacitor, particularly low value types of about 10µF or less. Low value electros are notoriously unreliable after a few years but higher values can also fail. It didn't take me long to confirm that the video output transistor collector voltages were very low indeed. This seemed to discount the capacitor theory because usually, when the bypass capacitor is open, a multimeter shows a rail voltage at somewhere around 50 to 75% of the normal voltage. During the flyback blanking period the rail rises to quite a high level because there is minimum load on the supply and no capacitance to absorb the supply pulse. Once blanking comes off, the video output transistors draw on every scrap of available energy and the rail voltage plummets. A multimeter averages these two levels and shows a rail voltage considerably higher than the true voltage during the line period. (Picture content also has a big effect on the rail voltage in the fault condition. A white screen loads the rail heavily, while a black screen draws little current and the rail can rise to almost normal value). In this case the voltage was very much lower than the condition described above, and also more consistent. This suggested that it was more likely that the pulse which generates the rail voltage was not even reaching the capacitor. And so it proved to be. R952, a 4 70 1W resistor feeding the capacitor (C104, 47µF), was reading several hundred kilohms and the surprising thing is that there was any voltage at all on the output collectors. And C104 was faultless . Hanover bars Replacing the resistor soon had the brightness back to normal and the set was displaying quite a reasonable picture. Except for one thing - severe Hanover bars. These bars are an obscure fault that occurs only in the PAL colour system. In PAL the phase of the transmitted chroma signal is alternated line by line and this is achieved by alternating the phase of the R-Y signal. So, if the phase of the red signal on line 1 is, say, 103°, then the same signal on the next line is shifted by 180° to - 77° (using the accepted chroma-phase scale), and so on down the picture. On playback, an identification signal is used to determine which of the lines is in the 103 ° phase and this line is displayed as is. The next line (at - 77°) has to be inverted, back to 103°. The circuitry used to perform this function is commonly referred to as the PAL switch. If this switching fails, the picture shows one line normal red, the next line (180° out) as cyan (bluish) and so on. The effect was first noted in Hanover, Germany, where PAL was developed, hence the name. It's sometimes called a venetian blind effect. TV TEST EQUIPMENT YOU NEED NOW!!! SHORTED TURNS TESTER (Built-in Meter) For checking EHT transformers including Split Diode Type, Yoke Windings and Drive Transformers. $78.00 + $3.00 p&p HI-VOLT AGE PROBE (Built-in Meter) Reads Positive or Negative 0-50kV Designed for checking TVs, microwave ovens and any high voltage equipment. $84.00 + $5.00 p&p TV TUNER REPAIRS Repair & Exchange (Philips ELC 2060) $17.00 + p&p Repair (Philips UV 461) $18.00 + p&p Repair & Exchange (Philips Turret) $18.00 + p&p Repair (AWA MITS Varicap) $18.00 + p&p Repair (Pye LM031 Turret) $18.00 + p&p P&P $2.00 Cheque, Money Order, Bankcard or Mastercard: Y.V.TuNERs) 216 Canterbury Road, Revesby, NSW 2212. Phone (02) 77 4 1154 ]UL Y 1989 53 I fed in a colour bar pattern from the generator and it showed quite distinct bars in the cyan, green, magenta and red sections of the pattern. The yellow bar was unmarked, a significant point that escaped my notice initially. Any set can show Hanover bars if the delay line settings are out of adjustment so the first thing I did was to go over all the adjustments as defined in the manual. This made no difference at all so I turned to the ident circuitry and looked for a more obscure answer. The PAL switching is carried out in the demodulator chip, IC804, and is driven by a multivibrator running at half line frequency. If the multivibrator starts up in the wrong phase, the reds and greens in the picture will be displayed in their complementary colours. To avoid this, an ident circuit looks at the instantaneous phase of the colour burst and develops a 7.8kHz pulse which steers the multivibrator into the correct phase. It was this circuitry that I looked at next. The manual gives instructions for adjusting the amplitude and phase of the multivibrator con54 SILICON CHIP trol circuit but I am sure that there is an error in them, although I haven't heard of any errata for this model. On page 18 the amplitude alignment calls for adjustment of T807 for maximum response as in waveform "B" which turns out to be a square wave with a curious bump on the top. Then on page 19 the next step uses the same T807 and is supposed to match the response curve "A", an H/ 2 sine wave. Even if waveforms "A" and "B" are swapped to make them agree with the instructions, the result is still ambiguous. The second adjustment is supposed to position the bump in the centre of the square wave. But this reduces the amplitude of waveform "A". If "A" is adjusted to maximum, as instructed, then the bump is shifted right off the end of the square wave. In the end I ignored the first instruction and settled for the centralised bump. This proved that the ident circuit was working but did nothing to alleviate the Hanover bars. Once I had satisfied myself that all the circuitry was working correctly, I began to suspect some subtle malfunction in one of the components - something that would give the right measurement but not process signals in the proper manner. So I started to change components. I had an old C1831X chassis that was known to be have been a good one so I lifted various parts and dropped them into the problem chassis. ICs, coils and transformers, transistors and even the chroma delay line all went into the villain, but nothing made any difference. By this time I was pretty desperate. The set had cost me nothing in cash but had piled up a considerable debt in hours. The pity of it all was that the set promised to show a very good picture, if only I could get rid of those (expletive) Hanover bars. And by this time I had another customer who wanted to pay cash for a good secondhand 44cm TV set; eg, a Sharp Cl831X. During all the preceding measuring and testing, I had frequently looked at the video waveforms on the output collectors. The blue waveform was the classic on-off shape and only proved that the blue channel was responding exactly as it should. The red and green on the other hand, were showing a curious sloping waveform that should have alerted me. One problem that comes from working on your own set is that the job is done in short bursts, when customer pressure eases off. So you often find yourself repeating something done several days earlier, and the time so wasted begins to pile up. Also, working so close to the problem for days and days tends to blind one to obvious clues. And so it was that I had to turn away the cash customer and put the old Sharp on the forget-about-it shelf. I had spent as much time on it as I was prepared to and the set would now have to be junked. It had cost me nothing in cash but 15 hours doesn't come cheaply even if it's my own time. A few weeks later, I was talking to a colleague about the set and its TETIA TV TIP Hitachi PA3-A Chassis (CEP288 etc) Symptom: l.,ow height. If height readjusted when the set is warm, it will still be low for a while after switch-on. Recovers height in two to five minutes. 20 volt rail varies from 1 6 to 1 8 volts during warmup but does not reach 20V. Cure: C753, a 1 0µ.F 25V electro defective. Capacity is very low Hanover bars. He is a college instructor and knows colour TV theory very intimately but he gets few chances to actually service a faulty set. He asked if he could have a look at it and I readily agreed. A few days later we set it up on his bench and the first thing he noticed was that the yellow bar was clear of the fault. This was no great shakes to me but to him it suggested that the ident was working at the beginning of each line but fading as the line progressed. And when he looked at the video output waveforms, the sloping characteristic, which I had noted but dismissed, confirmed this interpretation; the ident signal was losing its grip as each line scanned across the screen. · when cold and increases only slightly as the set warms up. This is the input capacitor to the 20V rail filter and its loss changes the filter to inductive input with consequent lower output. TETIA TV Tip is supplied by the Tasmanian branch of The Electronic Technicians' Institute of Australia. Contact Jim Lawler, 16 Adina St, Geilston Bay, Tasmania. There are no demodulator chip waveforms in the manual so one has to fall back on theory to determine what should be happening. The demodulator has to be supplied with chroma, a 4.43MHz reference signal, and a 7.BkHz ident signal. If all of these are present then the chip should produce the appropriate outputs. I had checked the presence of chroma and the 4.43MHz reference and had found what I thought was the ident signal. But I was wrong. That ain't right My friend confirmed the first two signals as present and correct but when he looked at the ident, on pin 11 of IC804, he said "Oy, that ain't right!" He was looking at a series of WE. C.OUL.t>N~ ) f'IIIJC> PN'lONE- ~~\~'t'e;.R ~P\N ~OU ~~ -~ e) ~!_l- 2~~~/\< A'RE'I'm GLA1) 11-\e:RE: 'SOME: ?eopt.£. S'R\GH-rER "ffi-~N N\£., O™ER vJ I-SE 1\-\5 WO'RL--.D WOU'-l> '8~ NE.C...K 1)€et> \N C> lJ O TE.L.-L-\.f 'S () ••. short pulses, first positive going then negative going, at 7.BkHz. These were what I had assumed to be the ident pulses but he knew that these short pulses could not properly control the phase reversal switch inside the chip. In fact, what is needed is a full square wave with a solid, steady voltage to hold the switch in whichever position it should be. The short pulse could only flip the switch but couldn't hold it in the right phase. Hence the clean yellow bar, on the left of the screen. This corresponded in time to the first few microseconds after the switch had changed over, before it drifted to an indeterminate position for the rest of the line. The square wave is also, in some way, responsible for setting the DC output levels of the R-Y and G-Y signals. In the fault condition, the pulse was enough to establish the right levels at the start of each line but then they drifted to some arbitrary level before the next pulse reset them. This explained the sloping waveforms I had noted in the red and green channels. In the end, the fault was so easy to find that it was ridiculous. It was C845, a 0.47 µ.F electro between the multivibrator output and the demodulator chip. On the capacitance bridge it read 270pF, far too little to pass a solid square wave. (It was apparently differentiating the original square pulses, converting them to spikes. Ed). As I said earlier, you can get too close to the job and miss the obvious clues. Then again, it helps to be the full bottle on theory when the manual offers no help. I thought the pulses only had to flip the switch, not hold it in position as well. I'm glad there are some people brighter than me, otherwise the world would be neck deep in dud tellys ! Well J. L., I've learned a lot from that and I'm sure our readers have too. I can't recall having encountered the Sharp 1831X, at least not in any complicated way, but I'll be on guard if I ever have to slide one out of the cabinet. I'll watch those wire wrap leads too. As for the Hanover bars - that was really nasty. No wonder it fazed you. (Ouch! - sorry about that).~ JULY 1989 55